Method and apparatus for an LNA with high linearity and improved gain control

ABSTRACT

A low noise amplifier (LNA) circuit includes a linear input stage including a first circuit that generates an output current and has a low input impedance. A device receives an input signal, communicates with the low impedance input of the first circuit, and includes a variable resistor having a resistance that varies based on the input signal.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation of U.S. patent application Ser. No.11/890,738, filed Aug. 7, 2007, which is a continuation of U.S. patentapplication Ser. No. 11/650,681, filed Jan. 8, 2007 (now U.S. Pat. No.7,253,690), which is a continuation of U.S. patent application Ser. No.11/435,995 (now U.S. Pat. No. 7,190,230), filed on May 17, 2006, whichis a continuation of Ser. No. 11/049,211 (now U.S. Pat. No. 7,088,187),filed on Feb. 2, 2005, which is a continuation of Ser. No. 10/242,879(now U.S. Pat. No. 6,977,553), filed on Sep. 11, 2002. The disclosuresof the above applications are incorporated herein by reference in theirentirety.

TECHNICAL FIELD

An aspect of the invention relates to Metal Oxide Semiconductor FieldEffect Transistor (MOSFET) amplifiers.

BACKGROUND

There is a growing demand for mobility in today's world. The rapidprogress in the wireless industry makes the ubiquitous connectionpossible. Radio Frequency (RF) transceivers are important components forwireless devices. The majority of the RF ICs used in the wirelesscommunication were implemented using either GaAs or silicon bipolartechnologies. Not until recently, when the continuous scaling of CMOStechnology brought the cutoff frequency (f_(T)) of MOS transistors up tomulti-tens of GHz, were such circuits built in CMOS technology possible.The advantage of using Complementary Metal-Oxide-Semiconductor (CMOS) RFis that it can be integrated with digital functions easily. As a result,it is possible to incorporate the whole system on one single chip whichyields low cost, small form factor wireless devices. A Low NoiseAmplifier (LNA) is an important building block in the wirelesstransceiver. For LNAs, the gain linearity applied to a signal is animportant operating characteristic, especially when the incoming signalis large. Under that condition, amplification by the LNA actually couldbe greater or smaller than one, and the noise contribution from the LNAmay be negligible compared to the input signal. In fact, the linearityof the LNA becomes the most important figure of merit. Gain linearity isgenerally characterized as a 1 dB compression point or third order InputIntercept Point (IIP3). The gain linearity is typically related to thetransconductance of a MOSFET in an input stage of the amplifier. Forexample, the transconductance of a MOSFET operating in the saturationregion is constant only when the input signal is small. When the inputsignal is large, the transconductance may vary as a function of theinput signal, leading to nonlinear amplification of the signal. Sourcedegeneration may be employed at lower frequencies to increase thelinearity of the input stage. However, at higher frequencies sourcedegeneration may not be effective due to the large parasitic capacitanceof the device. Also, source degeneration may increase power consumptiondue to the relative low gm/Id for the MOSFET in comparison with abipolar device. In addition, Gain control is also very important inpractical applications since the gain of the LNA could vary with processand temperature if not properly controlled.

SUMMARY

An LNA comprising an input stage to amplify an input signal. The LNAbeing particularly suitable for amplifying large input signals. Theinput stage includes a linearized transconductance and has reduced gainvariations in response to changes in process and environmentalconditions.

The details of one or more embodiments of the invention are set forth inthe accompanying drawings and the description below. Other features,objects, and advantages of the invention will be apparent from thedescription and drawings, and from the claims.

DESCRIPTION OF DRAWINGS

FIG. 1 is a block diagram of an aspect of a transceiver.

FIG. 2A is a schematic diagram of an aspect of an LNA.

FIG. 2B is a schematic diagram of an aspect of an LNA.

FIG. 2C is a schematic diagram of an aspect of an LNA.

FIG. 2D is a schematic diagram of an aspect of an amplifier.

FIG. 3 is a schematic diagram of an aspect of a bias circuit for alinear input stage.

FIG. 4 is a schematic diagram of an aspect of an amplifier.

FIG. 5 is a flow diagram of an aspect of an operation for generating alinear input stage.

FIG. 6 is a flow diagram of an aspect of an operation for biasing alinear input stage.

Like reference symbols in the various drawings indicate like elements.

DETAILED DESCRIPTION

FIG. 1 shows an aspect of a wireless transceiver 10 for communicatinginformation. The wireless transceiver 10 may include a Low NoiseAmplifier (LNA) 12 for amplifying an input signal. An input signal 14 tothe LNA 12 may be amplified by a linear input stage 18 constructed inaccordance with the principles of the invention. A bias circuit 16 maysupply bias signals to the linear input stage 18 in accordance with theprinciples of the invention. The LNA 12 preferably includes both thebias circuit 16 and the linear input stage 18. However, the LNA 12 mayinclude the bias circuit 16 combined with a conventional linear inputstage, or the linear input stage 18 combined with a conventional biascircuit. An output stage 20 may provide further amplification of theinput signal.

A mixer 22 may combine the amplified input signal with a Radio Frequency(RF) LO signal 24. A filter 26 and amplifier 23 may filter and amplifythe combined signal, and mix the generated signal with an IntermediateFrequency (IF) LO signal. An analog-to-digital converter (ADC) 28 mayconvert the mixed signal to a digital signal for further processing.

A digital-to-analog converter 27 may convert a digital signal to ananalog signal for transmission by a transmitter 25.

FIG. 2A shows an aspect of an LNA input stage 200 for amplifying aninput signal, v_(in). The LNA input stage 200 may be constructed usingany CMOS process including NMOS and PMOS. The input signal, v_(in), tothe LNA input stage 200 modulates the resistance of a first device 202that is connected to a second device 204 having a low input impedance.Due to the low input impedance of the second device 204, the voltagev_(x) at the junction of the first and second devices 202 and 204 mayremain relatively constant. The input stage 200 is configured so thatchanges in the input signal cause linearly proportional changes inconductance of the first device 202. In the case where v_(x) isrelatively constant and the conductance of the first device 202 changesin linear proportion to changes in the input signal, i_(out) is aboutlinearly proportional to v_(in).

FIG. 2B shows an aspect of an NMOS implementation 210 of the LNA inputstage 200. The resistance of a first device 212 is modulated in responseto an input signal v_(in). An NMOS transistor 214 in combination with anamplifier 216 provides a low impedance at the junction of the NMOStransistor 214 and the first device 212.

FIG. 2C shows an aspect of another NMOS implementation 220 of the LNAinput stage 210. Here, the resistance of a first NMOS transistor 222 ismodulated in response to an input signal v_(in). The first NMOStransistor is biased into the triode region. A second NMOS transistor224 in combination with an amplifier 226 provide a low impedance at thejunction of the first and second NMOS transistors 222 and 224.

FIG. 2D shows an aspect of an amplifier 30 for amplifying an inputsignal in accordance with the principles of the LNA input stage 200.Here, a linear input stage 32 may include an upper MOSFET, MB, 34 and alower MOSFET, MA, 36 connected in a cascode configuration. The inputimpedance of the upper MOSFET 34 at the junction of the upper and lowerMOSFETs 34 and 36, may be made low relative to the lower MOSFET 36 bycontrolling the relative sizes of the upper and lower MOSFETs 34 and 36.The linear input stage 32 is preferably constructed as an integratedcircuit using Complementary Metal Oxide Semiconductor (CMOS) technology,but other circuit technologies may also be used including discreteMOSFETs. Both NMOS and PMOS devices may be used. An input signal is ACcoupled through a capacitor 40 to the gate of the lower MOSFET 36. Abias circuit 38 biases the upper MOSFET 34 into the saturation regionand the lower MOSFET 36 into the triode region. Here, the lower MOSFET36 acts as a variable resistor changing conductance in linear proportionto changes in the input signal. The impedance of the junction of MOSFETs34 and 36 may be made lower by selecting the transconductance, g_(m), ofthe upper MOSFET 34 to be larger than both g_(ds) and g_(m) of the lowerMOSFET 36 so that Vds of the lower MOSFET 36 remains relatively constantover changes in the input signal. For example, an input switch ratiodefined as the ratio of the size of the upper MOSFET 34 to the size ofthe lower MOSFET 36 may be selected to be at least four, so that g_(m)of the upper MOSFET 34 is greater than both the g_(ds) and g_(m) of thelower MOSFET. One aspect of the invention recognizes that if the Vds ofthe lower MOSFET 36 is maintained relatively constant and the lowerMOSFET 36 is biased into the triode region, then the output current ofthe lower MOSFET 36 will be linearly proportional to the input signal.The following derivation illustrates that for a device in deep trioderegion:

${I_{d} = {\mu\; C{\frac{W}{L}\left\lbrack {{\left( {V_{gs} - V_{th}} \right)V_{ds}} - {\frac{1}{2}V_{ds}^{2}}} \right\rbrack}}},{g_{ds} = {\frac{\partial I_{d}}{\partial V_{ds}} = {{\mu\; C{\frac{W}{L}\left\lbrack {\left( {V_{gs} - V_{th}} \right) - V_{ds}} \right\rbrack}} \approx {\beta\left( {V_{gs} - V_{t}} \right)}}}},{{{where}\mspace{14mu}\beta} = {\mu\; C\frac{W}{L}}},$the output AC current is as follows:i _(out)=ν_(ds) g _(ds)=β(V _(gs) −V _(t))ν_(ds)Which shows that i_(out) may be a linear function of the input signal,leading to an increase in linearity. The amount of linearity achievedmay be controlled by adjusting the ratio of the upper MOSFET size to thelower MOSFET size. A load resistor 39 may be connected to the upperMOSFET 34. Another way of looking at it is to view the lower MOSFET 36as a normal MOSFET which has its own transconductance gm. The followingderivation illustrates that the linearity of g_(m) may be dependent onVds for a MOSFET operated in the triode region.

${I_{d} = {\mu\; C{\frac{W}{L}\left\lbrack {{\left( {V_{gs} - V_{th}} \right)V_{ds}} - {\frac{1}{2}V_{ds}^{2}}} \right\rbrack}}},$thus the transconductance of the device is,

$g_{m} = {{{\mu C}\frac{W}{L}V_{ds}} = {\beta\; V_{ds}}}$

The sensitivity of gm to variations in the input signal may be reducedby reducing the sensitivity of Vds to variations in the input signal,thereby increasing the linearity of the amplification.

However, since β is function of process and temperature variation, thegain of the amplifier may vary too. One way to reduce that sensitivityis to bias the input stage so that βVds is less sensitive toenvironmental variations.

FIG. 3 shows an aspect of a bias circuit 50 for a linear input stage.The bias circuit 50 may control the variation of the linear input stagetransconductance to reduce sensitivity to process, environmental effectssuch as temperature, and power. The bias circuit 50 includes an upperMOSFET, M2, 52 connected to a lower MOSFET M1, 54. The upper MOSFET 52is operated in the saturation region and the lower MOSFET is operated inthe triode region. A third MOSFET, M3, 56 operates to bias the lowerMOSFET 52 into the triode region. To set the bias to the lower MOSFET52, the magnitude of the current, I3, flowing through M3 56 may becontrolled as well as controlling the physical characteristics of M3 56such as size. For example, if I3 is selected to equal I1 (the currentflowing through M1), then a bias switch ratio defined as the ratio ofthe size of M1 54 to the size of M3 56 should be selected to be at leastgreater than one, and preferably greater than 1.4. A resistor 58connected from the gate of M1 56 decouples the input signal from thebias circuit 50.

FIG. 4 shows an aspect of an amplifier 59 including a bias circuit 60connected to a linear input stage 82. The bias circuit 60 is similar infunction to bias circuit 50 with corresponding elements in the range of62 to 68. The linear input stage 82 is similar in function to linearinput stage 32 with corresponding elements in the range of 84 to 86. Theamplifier 59 advantageously combines the benefits of both the linearinput stage 82 and the bias circuit 60. An input signal may be ACcoupled through a capacitor 60 to the gate of the lower MOSFET 86. Aload resistor 88 may be connected to the upper MOSFET 84 to obtain anoutput from the drain of the upper MOSFET 84.

The following derivation may be used to select the devices for thelinear input stage 82 and the bias circuit 60 of a preferred embodiment,and demonstrate how the gm of the input stage is controlled to be lesssensitive to environmental variations. The linear input stagetransconductance, g_(mA) may be as follows:

g_(mA)=βV_(ds,A)=β(V_(b)−V_(dsat,B)−V_(th,B)) where V_(b) is the voltagefrom the gate of MB to ground.

For discussion purpose, let's assume

${\left( \frac{W}{L} \right)_{2} = \left( \frac{W}{L} \right)_{B}},{\left( \frac{W}{L} \right)_{1} = \;\left( \frac{W}{L} \right)_{A}},$and I1=I2, then V_(dsa,2)=V_(dsat,B) and V_(th,2)=V_(th,B), thetransconductance of MA becomes;g_(mA)=β(V_(b)−V_(dsat,2)−V_(th,2))=βV_(ds,1). I1 is not limited to anyspecific ratio of 12 as long as the ratio of (W/L)₁ to (W/L)_(A) and(W/L)₂ to (W/L)_(B) are properly scaled so that the current densitiesare about the same for those devices. The ratio of the size of M2 to thesize of M1 should be approximately equal to the ratio of the size of MBto the size of MA.

For the same reason, let's assume I3=I1, and

${\left( \frac{W}{L} \right)_{1} = {X \cdot \left( \frac{W}{L} \right)_{3}}},$where X>1.0 and preferably 1.4. Then M1 is also in the triode region,and if M1 in deep triode region, Vgs−Vth>>Vds/2, thenI₁≈β(V_(gs,1)−V_(th,1))V_(ds,1)

$g_{m\; A} = {{\beta\; V_{{ds},1}} = {{\beta\frac{I}{\beta\left( {V_{{gs},1} - V_{{th},1}} \right)}} = {\frac{I_{3}}{\left( {V_{{gs},1} - V_{{th},3}} \right)} = {g_{m,3}/2.}}}}$If current I3 is a constant gm bias current which is;

${{Ids} = \frac{A}{\beta}},$where A can be chosen to only depend on an external resistor value andratio of two transistors[1], then,g_(mA)=g_(m,3)/2=√{square root over (2*I₃*β)}/2=√{square root over(A/2)} which is a constant.

Here, I3 does not have to equal I1, instead “X”, the ratio of the sizeof M3 to the size of M1, can be set to a predetermined value and theratio of I3 to I1 varied. Also, the ratios

$\left( \frac{W}{L} \right)_{1}$and

$\left( \frac{W}{L} \right)_{3}$may be varied to bias M1 into the triode region.

FIG. 5 shows an aspect of an operation for generating a linear inputstage. Starting at block 100, a semiconductor die is provided. At block102, a first MOSFET having a predetermined size is formed. At block 104,a second MOSFET having a size greater than the first MOSFET is formed.At block 106, the second MOSFET is connected in cascode with the firstMOSFET. At block 108, the first MOSFET is biased into the triode region.At block 110, the second MOSFET is biased into the saturation region. Atblock 112, an input signal is applied to the gate of the first MOSFETcausing a change in I_(d) of the MOSFETs that is approximately a linearfunction of the AC voltage applied to the first MOSFET gate.

FIG. 6 shows an aspect of an operation for biasing a linear input stage.Starting at blocks 120 and 122, first and second MOS devices areprovided. At block 124, the ratio of the first MOS device size to thesecond MOS device is selected to be a predetermined value, Rb. At block126, the second MOS device is connected in cascode with the first MOSdevice. At block 128, the first MOS device is biased into the trioderegion. At block 130, the second MOS device is biased into thesaturation region such as by connecting the gate and drain of the secondMOS device together. At block 132, a linear input stage having “A” and“B” MOS devices is provided. At block 134, the ratio of the “A” MOS sizeto the “B” MOS size is selected to be about Rb. At block 136, the firstand second MOS devices are connected to the linear input stage.

A number of embodiments of the invention have been described.Nevertheless, it will be understood that various modifications may bemade without departing from the spirit and scope of the invention.Accordingly, other embodiments are within the scope of the followingclaims.

1. A low noise amplifier (LNA) circuit, comprising: a linear input stageincluding: a first circuit that generates an output current and that hasa low input impedance; and a device that receives an input signal, thatcommunicates with the low impedance input of the first circuit, and thatincludes a variable resistor having a resistance that varies based onthe input signal.
 2. The LNA circuit of claim 1 further comprising abias circuit that biases the linear input stage.
 3. The LNA circuit ofclaim 2 wherein the bias circuit includes an amplifier.
 4. The LNAcircuit of claim 3 wherein the amplifier includes: a non-invertinginput; an inverting input that communicates with the low impedance inputof the first circuit; and an output that communicates with the firstcircuit.
 5. The LNA circuit of claim 1 wherein the first circuitincludes a transistor.
 6. The LNA circuit of claim 2 further comprisingan output stage that amplifies an output of the linear input stage.
 7. Awireless transceiver comprising the LNA circuit of claim 6 and furthercomprising: a mixer that receives an output of the output stage of theLNA circuit; and a filter that receives an output of the mixer.
 8. TheLNA circuit of claim 1 wherein the resistance of the device varies inproportion to the input signal.
 9. The LNA circuit of claim 1 whereinthe resistance of the device varies linearly with the input signal.